The present invention relates to improved high resolution/small field-of-view gamma camera systems for use in breast cancer and similar anatomical imaging, and more specifically, to shielding devices for such gamma detectors that eliminate the influence of stray/interfering radiation that may be present in the imaging environment.
X-ray mammography is the primary clinical screening tool for breast cancer. Over 15 million mammograms were performed in 1995 and over 25 million are expected to be performed annually by 2000. However mammography suffers from a high false positive rate. Currently, biopsies are performed following a positive mammogram to determine whether a suspicious lesion is cancerous or benign. Of the approximately 800,000 biopsies performed in 1995, roughly 600,000 were conducted on benign lesions. In addition to the expense involved, biopsy is a stressful procedure for the patient and the scarring left by the biopsy makes subsequent mammograms more difficult to interpret. Additionally, about 15-25% of all women have breast tissue that results in indeterminate mammograms. Dense tissue and scarring from prior surgery have x-ray densities similar to breast lesions, resulting in low contrast mammograms that are difficult to interpret.
Scintimammography has been shown to complement mammography by imaging the metabolic activity of cancerous lesions while ignoring benign lesions and healthy tissue. In studies conducted over the past five years involving 600 women, in connection with the approval process of the DuPont Merck Pharmaceutical Company imaging agent Miraluma(trademark), it was concluded that scintimammography is useful in differentiating cancerous and benign lesions. However, the studies also concluded that current large field-of-view gamma cameras cannot reliably image breast lesions smaller than 1.2-1.5 cm. In addition, the large size of these cameras limits their use to the lateral (side) views and does not allow for imaging the breast from other desirable viewing angles, and lesions in the chest wall are very difficult to detect.
To meet this need, so-called mini gamma cameras that are high resolution and image a small field-of-view have and are being developed.
A problem with these cameras, as with virtually all gamma emission detection based camera systems, is that they are prone to interference from extraneous radiation generated by sources other than the particular anatomical portion being examined. For example, in a typical breast examination, the patient is injected with a radiopharmaceutical and each breast imaged separately from a variety of positions by placing a gamma sensitive camera at various locations relative to the breast. In a lateral examination, the camera is placed outside of the breast under examination which results in its being xe2x80x9caimedxe2x80x9d not only at the breast under examination, but also at the opposing breast. Since the opposing breast may also have selectively absorbed some of the radiopharmaceutical, it is also emitting gamma radiation that may be detected by the camera aimed at the breast under examination. While gamma cameras conventionally include collimators whose purpose is to eliminate xe2x80x9cstrayxe2x80x9d radiation not impacting the camera at about a 90xc2x0 angle to the surface of the collimator, the location of the opposing breast at about 90xc2x0 to the collimator surface results in the collimator being relatively ineffective in eliminating this source of radiation. Similarly, when a caudal cranial view is taken (from below the breast) gamma emissions from radiopharmaceutical absorbed in the brain or thyroid may result in the production of similar interfering radiation that adversely affects the accuracy of the breast examination currently being performed.
Additionally, scatter radiation from tables and other structures in the environs of the imaging operation can result in the generation of impacting radiation that adversely affects the accuracy of the imaging operation. This problem is particularly acute in the case when the patient under examination is in the seated position as is often the case to enable the patient to remain motionless during the approximate 10 minute interval required to acquire the necessary image.
According to the present invention, there is provided an adjustable gamma radiation isolation shield that effectively shields the gamma camera from potentially interfering radiation emitted from sources other than the particular anatomical portion under examination. The shield is placed adjacent to the breast or other body portion under examination on the side thereof opposite that addressed by the gamma camera i.e., at about 90xc2x0 to the field of view of the camera, that is in a parallel plane to the plane of the camera. The shield comprises a first gamma ray permeable layer capable of absorbing fluorescent low energy radiation produced by a second adjacent gamma radiation absorbing layer. In operation, the breast is located between the gamma camera/detector and shield. Gamma radiation emitted by the radiopharmaceutical-containing breast impinges the gamma camera for measurement and simultaneously penetrates the gamma permeable layer and is absorbed by the gamma radiation absorbing layer. Fluorescence produced upon absorption of the gamma radiation is inhibited from reintroduction into the gamma camera by absorption in the first gamma permeable layer that absorbs this secondary radiation. Gamma or other potentially interfering radiation entering the system from beyond the gamma radiation absorbing layer is similarly absorbed and any secondary fluorescent radiation produced therein absorbed by the first gamma permeable layer. In this fashion, the gamma camera is effectively isolated from the potential impact of all extraneous radiation that might affect the accuracy or validity of the imaging operation.